“…For mothers, they often include vocalizations and gesture use [Jakubowski & Iverson, 2018; Talbott et al, 2016; Talbott, Nelson, & Tager‐Flusberg, 2015]. Recently, dyadic synchrony [Steiner, Gengoux, Smith, & Chawarska, 2018], responsiveness [Harker, Ibañez, Nguyen, Messinger, & Stone, 2016; Leezenbaum, Campbell, Butler, & Iverson, 2014; Schwichtenberg, Kellerman, Young, Miller, & Ozonoff, 2019], and indices of social engagement [Campbell, Leezenbaum, Mahoney, Day, & Schmidt, 2016; Harker et al, 2016; Kellerman, Schwichtenberg, Tonnsen, Posada, & Lane, 2019] have also been indexed. These diverse indices of prosocial behaviors make direct comparisons across studies difficult but general patterns of prosocial trajectories have emerged (described below), demonstrating distinct differences by familial ASD risk and outcome classifications within the first 2 years.…”